The present invention is directed to a catalytic structure that includes an active catalyst which is integral to the structure. The batch material of the structure is mixed with a catalyst, which is combined with a high surface area agglomerated body, added to the batch in a manner to preserve its chemical independence from the batch. The mixture is then extruded and fired or sintered to produce a structure with the catalyst integral to that structure.
Typically, a structure such as a ceramic honeycomb is batched, extruded, fired, coated with a catalyst/washcoat slurry, and then sintered to produce a catalytically active substrate. The product of such a procedure suffers from several disadvantages when put into use. Since the catalyst is integral only to the washcoat the residence time of the catalyst is dependent upon the integrity of the washcoat substrate bonding interface. When this product is subjected to the normal stresses of use the coating flakes off due to thermal expansion stresses, mechanical stresses due to effluent collisions, and vibrational shock.
An additional disadvantage relates to the production process itself. A coating operation is a multi-step process in which a substrate is produced, then coated, and finally sintered to the final product. Prior to being put into service the fragile coating is subject to mechanical stresses in the inevitable shipping and handling which must occur prior to the products final destination.
Prior attempts at resolving the problems inherent to the catalytically coated substrate have been directed at introducing powdered catalysts into the batch material, such as U.S. Pat. No. 4,295,818 (the '818 patent). The '818 patent mixes the catalyst with the batch material so that the catalyst is homogeneously interspersed throughout the structure.
The disadvantage with such a catalyst distribution is that necessarily, some of the active catalyst will be buried by the structure itself resulting in inaccessibility of certain portions of the active catalyst. Additionally, the catalyst may become bound to the matrix components of the structure. Unless the catalyst in some manner is able to accommodate the structural component, it may weaken the structure to subsequent degradation. This requirement of active catalyst and structural accommodation restricts the class of catalyst available to that particular catalyst-carrier system.
Other methods of integrating the catalyst into the structure of the catalyst carrier include impregnation, as in U.S. Pat. No. 4,522,940 and cosputtering as disclosed in U.S. Pat. No. 4,536,482. Both of these techniques are interposed after the substrate has been fired, therefore the high temperature reaction has occurred, leaving available bonding forces for the catalyst-carrier system that are orders of magnitude less than those if the catalyst is fired with the carrier.
Attempts have been made to integrate the washcoat material into the substrate as in U.S. Pat. No. 4,657,880 (the '880 patent). In the '880 patent the washcoat was integrated into the structure to provide a greater surface area for later incorporation of the catalyst material. This technique provided additional surface area for catalyst deposition, however, the catalyst was still not integral to the substrate structure.
The object of the present invention is to provide an active catalyst which is integral to the structure, avoid the disadvantages of the prior efforts exposed above, and to provide a simpler method by which to incorporate the catalyst into the structure of the catalyst carrier. A structure constructed as in the present invention may be advantageously placed in the exhaust path in an organically fueled power plant for catalysis of the noxious by-products of fuel combustion. The structure may also be usefully employed in the catalysis reactions required in the exhaust path of internal combustion engines or in any reactive system where a durable high surface area catalyst may be required.